 In vitro tumor models have become increasingly complex and diverse in recent years, with advances in cell biology, 3D cell culture, tissue engineering and micro fabrication enabling rapid development of new models that recapitulate critical steps in the metastatic cascade. These models are used for drug screening, patient specific therapies, and assessment of metastatic potential, but challenges remain in accurately modeling cancer recurrence and metastasis. This article was authored by Mariah E. Katt, Amanda E. Placone, Andrew E. Wong, and others.